.TH std::unordered_map 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::unordered_map \- std::unordered_map

.SH Synopsis
   Defined in header <unordered_map>
   template<

       class Key,
       class T,
       class Hash = std::hash<Key>,                                   \fB(1)\fP \fI(since C++11)\fP
       class KeyEqual = std::equal_to<Key>,
       class Allocator = std::allocator<std::pair<const Key, T>>

   > class unordered_map;
   namespace pmr {

       template<
           class Key,
           class T,
           class Hash = std::hash<Key>,
           class KeyEqual = std::equal_to<Key>                        \fB(2)\fP \fI(since C++17)\fP
       > using unordered_map =
             std::unordered_map<Key, T, Hash, KeyEqual,
                 std::pmr::polymorphic_allocator<std::pair<const Key,
   T>>>;

   }

   std::unordered_map is an associative container that contains key-value pairs with
   unique keys. Search, insertion, and removal of elements have average constant-time
   complexity.

   Internally, the elements are not sorted in any particular order, but organized into
   buckets. Which bucket an element is placed into depends entirely on the hash of its
   key. Keys with the same hash code appear in the same bucket. This allows fast access
   to individual elements, since once the hash is computed, it refers to the exact
   bucket the element is placed into.

   Two keys are considered equivalent if the map's key equality predicate returns true
   when passed those keys. If two keys are equivalent, the hash function must return
   the same value for both keys.

   std::unordered_map meets the requirements of Container, AllocatorAwareContainer,
   UnorderedAssociativeContainer.

.SH Notes

     * The swap functions do not invalidate any of the iterators inside the container,
       but they do invalidate the iterator marking the end of the swap region.
     * References and pointers to either key or data stored in the container are only
       invalidated by erasing that element, even when the corresponding iterator is
       invalidated.

.SH Template parameters

    This section is incomplete
    Reason: Add descriptions of the template parameters.

.SH Member types

   Member type              Definition
   key_type                 Key
   mapped_type              T
   value_type               std::pair<const Key, T>
   size_type                Unsigned integer type (usually std::size_t)
   difference_type          Signed integer type (usually std::ptrdiff_t)
   hasher                   Hash
   key_equal                KeyEqual
   allocator_type           Allocator
   reference                value_type&
   const_reference          const value_type&
   pointer                  std::allocator_traits<Allocator>::pointer
   const_pointer            std::allocator_traits<Allocator>::const_pointer
   iterator                 LegacyForwardIterator to value_type
   const_iterator           LegacyForwardIterator to const value_type
                            An iterator type whose category, value, difference, pointer
                            and
   local_iterator           reference types are the same as iterator. This iterator
                            can be used to iterate through a single bucket but not
                            across buckets
                            An iterator type whose category, value, difference, pointer
                            and
   const_local_iterator     reference types are the same as const_iterator. This
                            iterator
                            can be used to iterate through a single bucket but not
                            across buckets
   node_type \fI(since C++17)\fP  a specialization of node handle representing a container
                            node
                            type describing the result of inserting a node_type, a
                            specialization of

                            template<class Iter, class NodeType>
                            struct /*unspecified*/
   insert_return_type       {
   \fI(since C++17)\fP                Iter     position;
                                bool     inserted;
                                NodeType node;
                            };
                            instantiated with template arguments iterator and
                            node_type.

.SH Member functions

   constructor       constructs the unordered_map
                     \fI(public member function)\fP
   destructor        destructs the unordered_map
                     \fI(public member function)\fP
   operator=         assigns values to the container
                     \fI(public member function)\fP
   get_allocator     returns the associated allocator
                     \fI(public member function)\fP
.SH Iterators
   begin             returns an iterator to the beginning
   cbegin            \fI(public member function)\fP
   end               returns an iterator to the end
   cend              \fI(public member function)\fP
.SH Capacity
   empty             checks whether the container is empty
                     \fI(public member function)\fP
   size              returns the number of elements
                     \fI(public member function)\fP
   max_size          returns the maximum possible number of elements
                     \fI(public member function)\fP
.SH Modifiers
   clear             clears the contents
                     \fI(public member function)\fP
                     inserts elements
   insert            or nodes
                     \fI(since C++17)\fP
                     \fI(public member function)\fP
   insert_range      inserts a range of elements
   (C++23)           \fI(public member function)\fP
   insert_or_assign  inserts an element or assigns to the current element if the key
   \fI(C++17)\fP           already exists
                     \fI(public member function)\fP
   emplace           constructs element in-place
                     \fI(public member function)\fP
   emplace_hint      constructs elements in-place using a hint
                     \fI(public member function)\fP
   try_emplace       inserts in-place if the key does not exist, does nothing if the
   \fI(C++17)\fP           key exists
                     \fI(public member function)\fP
   erase             erases elements
                     \fI(public member function)\fP
   swap              swaps the contents
                     \fI(public member function)\fP
   extract           extracts nodes from the container
   \fI(C++17)\fP           \fI(public member function)\fP
   merge             splices nodes from another container
   \fI(C++17)\fP           \fI(public member function)\fP
.SH Lookup
   at                access specified element with bounds checking
                     \fI(public member function)\fP
   operator[]        access or insert specified element
                     \fI(public member function)\fP
   count             returns the number of elements matching specific key
                     \fI(public member function)\fP
   find              finds element with specific key
                     \fI(public member function)\fP
   contains          checks if the container contains element with specific key
   (C++20)           \fI(public member function)\fP
   equal_range       returns range of elements matching a specific key
                     \fI(public member function)\fP
.SH Bucket interface
   begin(size_type)  returns an iterator to the beginning of the specified bucket
   cbegin(size_type) \fI(public member function)\fP
   end(size_type)    returns an iterator to the end of the specified bucket
   cend(size_type)   \fI(public member function)\fP
   bucket_count      returns the number of buckets
                     \fI(public member function)\fP
   max_bucket_count  returns the maximum number of buckets
                     \fI(public member function)\fP
   bucket_size       returns the number of elements in specific bucket
                     \fI(public member function)\fP
   bucket            returns the bucket for specific key
                     \fI(public member function)\fP
.SH Hash policy
   load_factor       returns average number of elements per bucket
                     \fI(public member function)\fP
   max_load_factor   manages maximum average number of elements per bucket
                     \fI(public member function)\fP
                     reserves at least the specified number of buckets and regenerates
   rehash            the hash table
                     \fI(public member function)\fP
                     reserves space for at least the specified number of elements and
   reserve           regenerates the hash table
                     \fI(public member function)\fP
.SH Observers
   hash_function     returns function used to hash the keys
                     \fI(public member function)\fP
   key_eq            returns the function used to compare keys for equality
                     \fI(public member function)\fP

.SH Non-member functions

   operator==
   operator!=                    compares the values in the unordered_map
   \fI(C++11)\fP                       \fI(function template)\fP
   \fI(C++11)\fP(removed in C++20)
   std::swap(std::unordered_map) specializes the std::swap algorithm
   \fI(C++11)\fP                       \fI(function template)\fP
   erase_if(std::unordered_map)  erases all elements satisfying specific criteria
   (C++20)                       \fI(function template)\fP

     Deduction guides \fI(since C++17)\fP

.SH Notes

       Feature-test macro       Value    Std                   Feature
   __cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for
                                               containers

.SH Example


// Run this code

 #include <iostream>
 #include <string>
 #include <unordered_map>

 int main()
 {
     // Create an unordered_map of three strings (that map to strings)
     std::unordered_map<std::string, std::string> u =
     {
         {"RED", "#FF0000"},
         {"GREEN", "#00FF00"},
         {"BLUE", "#0000FF"}
     };

     // Helper lambda function to print key-value pairs
     auto print_key_value = [](const auto& key, const auto& value)
     {
         std::cout << "Key:[" << key << "] Value:[" << value << "]\\n";
     };

     std::cout << "Iterate and print key-value pairs of unordered_map, being\\n"
                  "explicit with their types:\\n";
     for (const std::pair<const std::string, std::string>& n : u)
         print_key_value(n.first, n.second);

     std::cout << "\\nIterate and print key-value pairs using C++17 structured binding:\\n";
     for (const auto& [key, value] : u)
         print_key_value(key, value);

     // Add two new entries to the unordered_map
     u["BLACK"] = "#000000";
     u["WHITE"] = "#FFFFFF";

     std::cout << "\\nOutput values by key:\\n"
                  "The HEX of color RED is:[" << u["RED"] << "]\\n"
                  "The HEX of color BLACK is:[" << u["BLACK"] << "]\\n\\n";

     std::cout << "Use operator[] with non-existent key to insert a new key-value pair:\\n";
     print_key_value("new_key", u["new_key"]);

     std::cout << "\\nIterate and print key-value pairs, using `auto`;\\n"
                  "new_key is now one of the keys in the map:\\n";
     for (const auto& n : u)
         print_key_value(n.first, n.second);
 }

.SH Possible output:

 Iterate and print key-value pairs of unordered_map, being
 explicit with their types:
 Key:[BLUE] Value:[#0000FF]
 Key:[GREEN] Value:[#00FF00]
 Key:[RED] Value:[#FF0000]

 Iterate and print key-value pairs using C++17 structured binding:
 Key:[BLUE] Value:[#0000FF]
 Key:[GREEN] Value:[#00FF00]
 Key:[RED] Value:[#FF0000]

 Output values by key:
 The HEX of color RED is:[#FF0000]
 The HEX of color BLACK is:[#000000]

 Use operator[] with non-existent key to insert a new key-value pair:
 Key:[new_key] Value:[]

 Iterate and print key-value pairs, using `auto`;
 new_key is now one of the keys in the map:
 Key:[new_key] Value:[]
 Key:[WHITE] Value:[#FFFFFF]
 Key:[BLACK] Value:[#000000]
 Key:[BLUE] Value:[#0000FF]
 Key:[GREEN] Value:[#00FF00]
 Key:[RED] Value:[#FF0000]

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to          Behavior as published              Correct behavior
                       the definitions of reference,
   LWG 2050 C++11      const_reference, pointer                 based on value_type and
                       and const_pointer were based on          std::allocator_traits
                       allocator_type

.SH See also

   map collection of key-value pairs, sorted by keys, keys are unique
       \fI(class template)\fP

.SH Category:
     * Todo with reason
